Cant you simply replace the MCB with an RCBO?
Im not a qualified sparky, but would do it in the consumer unit, either with a RCBO or feeding the MCB with and additaional/existing RCd.
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Get it done properly at the consumer unit.
install the RCD protection at the consumer unit, either by a type tested RCBO or an external shower CU.
an RCBO in the CU is the way to go.
fit a separate RCD in an enclosure next to the cu
If your consumer unit will accept an RCBO
That about sums it up.